The effect of sympathetic neural tone on certain electrophysiologic changes occurring in the heart during and after intermittent occlusion of a coronary artery will be studied in anesthetized open chest dogs. Electrophysiologic items to be measured include changes in myocardial conduction times and refractory periods within and outside the ischemic lesion. Special attention will be given to changes occurring at the margin of the ischemic lesions. Conduction times will be measured using tangentially and transmurally placed multi-electrode needles, usings up to 60 simultaneously recorded sites. The multiplexed data will be recorded on analogue tape and computer analyzed. Refractory periods will be determined during and after the same or repeat coronary artery occlusion. Cardiac sympathetic tone will be varied using nerve section, direct stimulation of cardiac sympathetic nerves, and drugs. Results are expected to provide new information concerning the effects of sympathetic tone on certain physiologic properties which are known to change during ischemia and which may be important in the origin of arrhythmias associated with ischemic heart disease. BIBLIOGRAPHIC REFERENCE: Abildskov, J. A.: The sequence of normal recovery of excitability in the dog heart. Circulation 52:442-446, 1975.